The present invention relates to an independent ignition type ignition coil for an internal combustion engine which is installed in a plug hole of the engine to be directly coupled with each spark ignition plug.
Since such independent ignition type ignition coil is introduced at least a part of a coil portion within a plug hole and installed therein, a center core (in which a plurality of silicon steel plates are stacked on a magnetic path iron core), a primary coil and a secondary core are housed with a thin cylindrical coil casing. A high voltage necessary for spark ignition is generated in the secondary coil by controlling supply and block of a current for the primary coil. These coils are normally wound on respective bobbins and are arranged around the center core in coaxial fashion.
As the ignition coil of this kind, there are a so-called outer secondary coil structure, in which the primary coil is arranged inside and the secondary coil is arranged outside, and a so-called inner secondary coil structure, in which the secondary coil is arranged inside and the primary coil is arranged outside. Amongst, the latter is considered to be advantageous in comparison with the former in view point of output characteristics for shorter overall length of the secondary coil in comparison with the former and smaller electrostatic stray capacitance.
Namely, a secondary voltage output and a rising characteristics thereof are affected by the electrostatic stray capacitance to lower the output and to delay rising at greater electrostatic. Accordingly, the secondary coil having smaller electrostatic stray capacitance is considered to be more suitable for down-sizing and higher output.
Within the coil casing housing the primary and secondary coils, insulation ability of the coils is assured by filling an insulative resin (filled and cured).
However, when an epoxy resin is filled (filling and curing) between the components of the ignition coil assembly, since curing temperature of the epoxy resin is typically higher than or equal to 100xc2x0 C., and at normal temperature, the insulative resin and bobbin material are exerted thermal stress due to different of linear expansion coefficients of the components (difference of linear expansion coefficients of bobbin, coil, center core and the insulative resin). Thus, it becomes necessary to provide a measure for preventing crack and interfacial delamination between the materials due to thermal stress.
In Japanese Patent Application Laid-open No. Heisei 11-111545, there has been disclosed an ignition coil of inner secondary coil structure, in which the insulative resin is filled (filled and cured) within a coil casing housing therein the primary and secondary coils. On the other hand, there has been disclosed that even if the resin insulation material penetrates between the wire of the primary coil, sliding may be caused the wire of the primary coil and the resin insulation material by coating wire of the primary coil by a material which is difficult to be bonded by the insulative resin to be filled.
However, in the prior art, when the primary coil and the insulative resin are tightly fitted, the surface of the primary coil can be scratched by the insulative resin to cause peeling off of the coating.
It is an object of the present invention to make an ignition coil assembly of this kind high quality and high reliability by reducing a thermal stress due to a difference of linear expansion coefficients (difference of linear expansion coefficients of bobbin, coil, center core and insulative resin) between components without causing break down of electrical insulation of a primary coil.
In order to accomplish the above-mentioned object,
(1) Namely, the first invention is that in an independent ignition type ignition coil for an internal combustion engine to be used by directly connecting with each ignition plug of the internal combustion engine, in which a center core, a secondary coil wound around a secondary bobbin, and a primary coil wound around a primary bobbin are coaxially arranged in sequential order from inside within a coil casing, and insulative resin being filled between these components,
a gap portion for reducing stress generated within the secondary bobbin by thermal shrinkage difference of the first coil and the secondary bobbin among thermal stress created within the secondary bobbin, is provided together with the insulative resin between the primary bobbin and the primary coil and between the layers of the primary coil.
This gap is at least one delaminated portion formed between xe2x80x9cthe insulative resin (for example, epoxy resin) filled between the primary bobbin and the primary coilxe2x80x9d and the xe2x80x9cprimary coilxe2x80x9d, between the xe2x80x9cinsulative resin filled between the primary bobbin and the primary coilxe2x80x9d and the xe2x80x9cprimary coilxe2x80x9d, and between the xe2x80x9cprimary coilxe2x80x9d and xe2x80x9cinsulative resin filled between layers of the primary coilxe2x80x9d.
As more particular mode of implementation, the primary coil is provided with the coating film or coating which is easy to delaminate between the primary coil and the insulative resin filled around the primary coil, is provided with the coating film or coating easy to delaminate between the bobbin surface and the insulative resin contacting on the bobbin surface, and is provided, in place with the coating film or coating, with an insulative sheet having low bonding ability with epoxy.
As these coating film or coating, overcoating containing material having small friction coefficient, such as nylon, polyethylene, Teflon or the like and material having small bonding ability with epoxy resin is used.
After curing epoxy, when temperature is lowered, delamination is caused in the portion having small tension stress at the interface between epoxy and the primary coil or the primary bobbin and small bonding ability with epoxy, due to difference of linear expansion coefficients of copper and epoxy.
As an effect of the present invention, when thermal shrinkage is caused in the ignition plug by lowering of temperature after stopping operation of the engine, relative expansion force in circumferential direction acts on the secondary bobbin by thermal shrinkage difference (linear expansion coefficient difference). On the other hand, from the primary coil and the secondary coil, tension force acts on the secondary coil relatively in circumferential direction via the insulativeresin. By multiplier effect of these, large internal stress "sgr" is created in the secondary bobbin. In the present invention, by interposing the gap (for example, the foregoing delaminated portion) between the primary bobbin and the primary coil and/or between the layers of the primary coil, it becomes possible to block transmission path of the tension force in the circumferential direction otherwise acting on the secondary bobbin from the primary coil.
Accordingly, among the stress "sgr" created within the secondary bobbin, by reducing the stress component "sgr"1 created within the secondary bobbin by thermal shrinkage difference of the primary bobbin and the secondary bobbin, total internal stress "sgr" can be significantly reduced (weaken). By examples of CAE (Computer Aided Engineering) analysis made by the inventors, by reducing the foregoing stress component "sgr"1, at least 20% of the total internal stress can be reduced. The reduction value of the internal stress was confirmed in connection with the ignition coil inserted into the plug hole of the internal combustion engine to be directly connected to the ignition plug, and the outer diameter of the inserted portion is xc3x818 to xc3x827 mm (the thin cylindrical type ignition coil of this size typically has 0.5 to 1.2 mm of thickness of the primary bobbin, 0.7 to 1.6 mm of thickness of the secondary bobbin, and 50 to 150 mm of bobbin length).
Even when the foregoing gap (for example, laminated portion) is provided between the primary bobbin and the primary coil and/or between the layers of the primary coil, since the primary coil is low potential (substantially ground potential), concentration of electric field between the primary coil will never be caused. Also, by tightly fitting the secondary coil, the insulative resin and the primary bobbin without gap, insulation between the primary coil and the secondary coil can be sufficiently assured. It has also been confirmed by the result of test that concentration of electric field by line voltage of the secondary coil can be satisfactorily prevented. Thus, insulation break down can be prevented.
(2) Furthermore, in addition to the foregoing first invention, when modified PPE (modified polyphenylene ether) is used for the secondary bobbin, the internal stress "sgr" can be further reduced in viewpoint of improvement of material of the secondary bobbin by containing inorganic filler (glass fiber, Mica, Talk or the like) in the content of greater than or equal to 20% in the secondary bobbin.
Modified PPE is superior in bonding ability with epoxy resin serving as the insulative resin, and has good molding ability and insulation ability. Therefore, it can contribute for quality stability of the secondary bobbin. When the inorganic filler content is less than 20%, the difference of linear expansion coefficients with other component (center core, primary coil, secondary coil or the like) becomes large to make the internal stress (thermal stress) "sgr" large, For example, according to the example of CAE analysis, if the foregoing "sgr"1 is not reduced, the internal stress "sgr" created in the secondary bobbin becomes as large as about 90 to 100 MPa upon occurrence of abrupt temperature drop if the ignition coil is placed in temperature environment varying from 130xc2x0 C. to xe2x88x9240xc2x0 C.
In contrast to this, according to the present invention, the internal stress "sgr" can be lowered to be less than or equal to 70 MPa to successfully prevent longitudinal cracking of the secondary bobbin. It should be noted that as optimal example of lowering of the internal stress "sgr" with maintaining bolding ability (flowability of the resin) of the secondary bobbin, it is proposed a material containing 45 to 60 Wt % of modified PPE, 15 to 25 Wt % of glass fiber, 15 to 35 Wt % of non-fibric inorganic filler. The detail will be discussed in the discussion of the embodiment.
Furthermore, in viewpoint of the linear expansion coefficient lowering the foregoing internal stress "sgr", particularly, when resin flow direction in resin molding is axial direction of the bobbin, the linear expansion coefficient in the direction perpendicular to the resin flow direction (it becomes important point for preventing longitudinal cracking of the bobbin to suppress internal stress in the direction corresponding to radial direction and circumferential direction of the bobbin, particularly in circumferential direction) of 35 to 75xc3x9710xe2x88x926 in average at xe2x88x9230xc2x0 C. to xe2x88x9210xc2x0 C. in test method according to ASTM D 696. Detail of this will be discussed in the discussion of the embodiment.
As more particular embodiment, by forming the coating film or coat layer on the outermost layer of the primary coil containing component having no affinity or causing no chemical reaction with the insulative resin (for example, epoxy resin), delamination is caused between the primary coil and the insulative resin to form the gap portion. The component having no affinity or causing no chemical reaction with the insulative resin is the material expressed by "Parenopenst"CH2CH2"Parenclosest"n(nxe2x89xa72) or "Brketopenst"CH2xe2x80x94CH(CH3)"Brketclosest"n(nxe2x89xa72), for example, nylon, polyorefin such as polyethylene, polypropylene or the like, fruorinated resin, fluorinated ester, fluorinated rubber, wax, fatty acid ester.